| Graphitic carbon nitride(g-C3N4),consisting of C and N elements,is an organic polymer semiconductor with a smaller band gap of 2.7 e V,which exhibits visible light activity(?< 460 nm).In addition,its abundant precursors,simple preparation methods and stable physicochemical properties render it received more and more attention.However,the pristine g-C3N4 shows very lower solar energy utilization efficiency due to smaller surface areas,serious photon-generated carrier recombination,limited visible light absorption range.Morphological engineering to increase the surface area,element doping to extend the light absorption range and accelerate carriar migration,heterojunction structure to improve the carriar separation,and the local surface plasmon resonance(LSPR)to increase light absorption range are all proved to be effective methods to improve the photocatalytic activity of g-C3N4.Besides,developing the supramolecular precursors,which is composed of melamine and cyanuric acid by hydrogen bonding self-assembly,could keep the morphology at high temperatures.In the meantime,the interactions between hydroxyl,carboxyl and other molecules and ions are beneficial to morphology engineering and elemental doping of g-C3N4.In this dissertation,the g-C3N4 photocatalysts based on the supramolecular precursor from melamine and cyanuric acid,have been designed and prepared for morphological engineering,element doping,plasmonic photocatalysts and improving the charge channels through carbon-bridge.And photocatalytic activities for hydrogen evolution have been improved,and the preliminary photocatalytic mechanisms are also suggested.A series of works have been finished:1.Aiming at tedious preparation procedures for 2D porous graphitic carbon nitride(2DPCN)nanosheets,a one-step thermal polymerization method based on a supramolecular precursor had been developed.As-prepared 2DPCNs exhibited enlarged specific surface areas,better crystallinity,broader visible-light harvesting capability,aligned energy levels and faster charge transfer.A superior photocatalytic hydrogen evolution rate of 220 ?mol h-1 under visible-light(? > 420 nm)irradiation had been achieved.In the meantime,an apparent quantum yield(AQY)of 1.3% at 490 nm was also obtained,indicating that 2DPCN possesses green light activity.Our one-step thermal polymerization method based on a supramolecular precursor provided a new way to develop g-C3N4 photocatalysts with different morphologies and better photoelectric properties.2.Sodium-doped carbon nitride nanotubes(Nax-CNNTs)were prepared by a green and simple two-step method and applied in photocatalytic water splitting for the first time.Element mapping and X-ray photoelectron spectroscopy(XPS)measurements confirm that sodium was successfully introduced in the carbon nitride nanotubes(CNNTs),and the intrinsic structure of g-C3N4 was also maintained in the products.Moreover,the porous and nanotube structure of the CNNTs leaded to relatively large specific surface areas.Photocatalytic tests indicated that the porous tubular structure and Na doping could synergistically enhance the hydrogen evolution rate under visible light(? > 420 nm)irradiation in the presence of sacrificial agents,leading to a hydrogen evolution rate as high as 143 ?mol·h-1(20 mg catalyst).Moreover,other alkali metal-doped CNNTs,such as Lix-CNNT and Kx-CNNT were tested;both materials were found to enhance the hydrogen evolution rate.This highlights the general applicability of the present method to prepared alkali metal-doped CNNTs;a preliminary mechanism for the photocatalytic hydrogen evolution reaction in the Nax-CNNTs was also proposed.3.Au nanorods(NRs)decorated carbon nitride nanotubes(Au NRs/CNNTs)photocatalysts had been designed and prepared by impregnation–annealing approach.Localized surface plasmon resonance(LSPR)peaks of Au NRs could be adjusted by changing the aspect ratios,and the light absorption range of Au NRs/CNNTs was extended to longer wavelength even near-infrared light.Optimal composition of Pt@Au NR769/CNNT650 had been achieved by adjusting the LSPR peaks of Au NRs and further depositing Pt nanoparticles(NPs),and the photocatalytic H2 evolution rate was 207.0 ?mol h-1(20 mg catalyst).Preliminary LSPR enhancement photocatalytic mechanism was suggested.On one hand,LSPR of Au NRs was beneficial for visible-light utilization.On the other hand,Pt NPs and Au NRs had a synergetic enhancement effect on photocatalytic H2 evolution of CNNTs,in which the local electromagnetic field could improve the photogenerated carrier separation and direct electron transfer increase the hot electron concentration while Au NRs as the electron channel can well restrain charge recombination,finally Pt as co-catalyst can boost H+ reduction rate.This work provides a new way to develop efficient photocatalysts for splitting water,which can simultaneously extend light absorption range and facilitate carrier generation,transportation and reduce carrier recombination.4.A new carbon-bridge connected heptazine-based melon(CCHM)had been designed to facilitate the charge transfer,which was derived from calcining a new supramolecular precursor based on the hydrogen bond self-assembly of melamine,cyanuric acid and oxalic acid in water.Under visible-light(? > 420 nm)irradiation,the hydrogen evolution rate of 106 ?mol h-1 had been achieved and the AQY of CCHM was also estimated to be 13.1% at 420 nm.The theoretical calculation and experiment indicated the carbon-bridge form the charge channel between melons,which could improve the charge transfer,restrain carrier recombination and improve the body charge utilization,thus boosting the photocatalytic activity.This work provided a new and facile way to improve the charge transfer of heptazinebased melon and the photocatalytic activity as well. |
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